Topic 16

Nature of Matter - II



During the 19th Century formalized experimentation led to a plethora of new observations about matter. One special method for improving our understanding of matter was to heat various elements (mostly metals) up in a vacuum and measure the frequency of light that was given off.

It turns out that each natural substance (element) gives off light at a series of frequencies special to that material as they are heated up.

Spectroscopy is the scientific method of measuring those individual frequencies of light in the laboratory.


Cathode Rays

As materials were being heated up in a vacuum and their light frequencies measured, some were observed to give off a visible radiation which would not penetrate to glass of the vacuum chamber and thus could not be light.

This radiation was termed Cathode Radiation.


Discovery of Electrons

J.J. Thompson studied cathode rays by placing electrical or magnetic field around the vacuum chamber and noting the effects of changing their configuration or strength.

He observed that cathode rays were deflected by electrical fields.

From this he determined that the cathode rays were negatively charged particles.

Using his equipment, he could measure the ratio of electron charge (e) to is mass (m) and found that all cathode rays for a single material had the same ratio e/m.

He next found that different materials all had the same value of e/m. From this he determined that the cathode rays or ELECTRONS were a fundamental part of the atoms of all materials.


Discovery of Ions

J.J. Thompson also noticed that if a little gas is placed in the vacuum chamber with cathode rays that another type of invisible particle is formed.

He argued that these particles form by the breakdown of gas by hitting them with electrons

He called them ions.

All were positively charged but had a variety of e/m ratios. (This is due to presence of stable isotopes, but he didn't know that.)


Thompson Atom

On the basis of his experimentation, Thompson argued that atoms are composed of equal amounts of particles with positive and negative charge.

The atom looks like a watermelon with negative electrons being the seeds and positive ions being the main part of the watermelon. (Later it was found that ions have positively charged protons and neutrons with no charge.)



Wilhem Roentgen (1895) noted that when cathode rays hit some other solid material, a new form of invisible radiation is given off. He noted the radiation because it caused a film plate to darken or a fluorescent screen to fluoresce. The film plate was lined with lead so normal radiation like visible light could not penetrate it.

He called the radiation x-rays. We now know that x-rays are electromagnetic radiation with very high frequency and energy so that it will pass through solid material.


Natural Radioactivity

In a similar manner, some substances, when coated on a film plate with cause it to cloud without any other activity.

Put these substances in a vacuum and without doing anything to them, they emit ions, electrons (cathode rays), and gamma-rays (another form of electromagnetic radiation with even shorter frequency and higher energy).


Rutherford Atom

Rutherford bombarded metal foils with ions and noted that almost all went through or caused ejection of electrons.

Only on rare occasion did an ion get bounced back from the metal foil.

From this Rutherford deduced that the atom must contain a lot of empty space with electrons and the positive ions must be at the center in a nucleus.

From experiments, we now estimate the normal size of a nucleus to be 1/100,000 the diameter of an entire atom!

Electrons revolve around nucleus in orbits - why don't they give up energy and spiral into nucleus?


Blackbody Radiation

A blackbody is simply an experimental apparatus that will absorb all radiation incident on it and re-radiate energy in a range of frequencies.

Classical physics argued that the intensity of blackbody radiation should increase with higher frequency. The reality is that there is almost no radiation at high frequency. Why?


Planck's Constant

Planck was the first to find a single equation that could predict the actual pattern of radiation out of a blackbody.

Problem was to figure out why the equation worked.

He used statistical mechanics and the notion of Entropy to a fundamental conclusion

E = hn where h is called Plank's constant and n is light frequency.

Planck's constant describes the size of packages of energy in light.

h=6.55 x 10-27 erg seconds

'erg seconds' is an odd set of units - called an action.

Action is only definable in 4-space (but special theory of relativity was not yet developed).


Einstein and the Photoelectric Effect

Thompson and Leonad noted that light of one frequency caused electrons to be emitted from metal foil in a vacuum.

Increasing light intensity did not affect the system, but changing frequency caused electrons to be emitted at higher energy.

Einstein explained this in terms of light quanta and energy.


Bohr's Atom - A New Paradigm

Bohr combined quantization of energy with the Rutherford Atom to develop a new view of the atom. This was a true new paradigm - able to explain a wide body of observations and serve to predict things.

The model fit all previous observations to a first approximation.

Model made predications for spectroscopy and chemistry.

Model is still used today in introductory texts even in college.

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